The #undef Statement
The #undef statement is used to undefine macro variables. It has the following syntax:
# undef $variable
$variable
Specifies the name of the macro variable you want to undefine. You can specify only one variable in each #undef statement.
You cannot undefine a built-in macro variable. If you use #undef for a macro variable that does not currently exist, the statement is ignored and processing continues.